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Re: More semantic comments (XML Schema value spaces)

From: pat hayes <phayes@ai.uwf.edu>
Date: Wed, 13 Nov 2002 14:20:37 -0500
Message-Id: <p05111b0bb9f853856c2b@[10.0.1.100]>
To: Dan Connolly <connolly@w3.org>
Cc: Jeremy Carroll <jjc@hplb.hpl.hp.com>, w3c-rdfcore-wg@w3.org, "Henry S. Thompson" <ht@cogsci.ed.ac.uk>

>[+cc Henry in case he can clear this up quickly.
>If not, Henry, don't feel obliged to reply; I'll
>eventually report my problem with this
>to www-xml-schema-comments]
>
>On Wed, 2002-11-13 at 08:13, pat hayes wrote:
>>  >On Mon, 2002-11-11 at 17:13, pat hayes wrote:
>>  >[...]
>>  >>  >3.4
>>  >>  >
>>  >>  >I found the following unintelligible, possibly because I don't have
>>  >>  >access to the XSD example in your (or mayber Peter's) head.
>>  >>  >
>>  >>  >[[Users shoudl take care to distinghuish the value space ...
>>  >>  >identical when viewed as class members.]]
>>  >>
>>  >>  Yes, it is rather odd. The plain fact is that the XML schema specs
>>  >>  are logically contradictory when one thinks of a value space as a
>>  >>  set. Enquires have determined that XMLSchema value spaces are not
>>  >>  sets,
>>  >
>>  >?!?!?
>>  >
>>  >yes, they are:
>>  >
>>  >"A value space is the set of values for a given datatype."
>>  >  -- http://www.w3.org/TR/2001/REC-xmlschema-2-20010502/#value-space
>>
>>  There is little point in quoting one assertion from a text that has
>>  internal contradictions.
>
>I wasn't aware there were internal contradictions.
>I'd be interested in help finding the text that
>contradicts the above.
>
>>  But I spoke too hastily. What I should have said is that they are not
>>  sets *of the values which the spec declares them as containing*. The
>>  spec (plus published or just-to-be-published errata) insists that no
>>  two primitive value spaces have the same members, by definition.
>
>!?!?!
>
>I was a member of the WG, and I remember no such decision.

I gather that there is a conceptual distinction between value *space* 
and the set of members of the value space. The spaces are considered 
to have distinct members, even if the sets are the same.

>
>I guess I'll have to watch the errata.
>
>I don't see any relevant ones so far.
>   http://www.w3.org/2001/05/xmlschema-errata
>   $Id: xmlschema-errata.html,v 1.16 2002/11/04 15:05:09 ht Exp $
>
>>  It
>>  also says that base64Binary and hexcodedBinary both have value spaces
>>  containing the same members.
>
>yes, that's clear enough:
>
>"The ·value space· of hexBinary is the set of finite-length sequences of
>binary octets."
>
>"The ·value space· of base64Binary is the set of finite-length sequences
>of binary octets."
>
>
>>  On being queried about this, Henry
>>  Thompson gave the following replies: (1) primitive value spaces can
>>  contain the same members (2) members of distinct primitive value
>>  spaces are considered to be distinct, by definition.  This is
>>  impossible in set theory, no matter what the spec says. The spec is
>>  evidently confused.
>
>Something or somebody is confused; I'm not convinced it's
>the text of the spec.

Well, maybe not. But I no longer have any confidence that I know what 
the text is intended to mean.

>
>>  >  > but something like categories or algebras. The same thing, seen
>>  >>  as a member of two different value spaces, is considered to be two
>>  >>  different things.
>>
>>  That is the impression I formed from trying to make sense of what
>>  Henry said and re-reading the spec in that light. But it is only an
>>  impression. Another interpretation is that the spec uses a
>>  nonstandard notion of 'value' in which there are two layers of
>>  interpretation and the xsd-"value' is the thing in the first layer,
>>  while the RDF-mathematical-"value" is in the second layer, with a
>>  many-to-one mapping between them.
>
>Blech.

I agree.

>
>>  >Inquiries may *suggest* otherwise, but only excerpts from
>>  >the text of the spec can *determine* what XML Schema
>>  >value spaces are.
>>  >
>>  >I'm interested in what leads you to think they're not
>>  >sets.
>>
>>  Extended series of emails from Henry Thompson in reply to a direct
>>  query on the topic from Brian, followed by searching enquiries from
>>  me. I can forward them if you like. Henry's view is that the value
>>  spaces are best seen as sets of pairs <value, value space>. These are
>  > not the same sets as the class extensions of the datatype domains in
>>  RDF.
>
>Eek! Train wreck approaching!

I said exactly the same to Henry.

Now, to be fair, Henry did say quite explicitly that it would be fine 
for us, ie RDF, to say that two different primitive datatype classes 
have the same members. But he also wants to reserve the right for 
XMLS to say that the corresponding value spaces do NOT have any 
members in common. Seemed to me that this is likely to produce some 
misunderstandings if left unremarked on, so I put in some warning 
prose.

Pat

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Received on Wednesday, 13 November 2002 14:20:13 EST

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